1. Field of Invention
The invention relates generally to coordinate measuring machines, and more specifically to methods and apparatuses for calibrating a probe tip of a coordinate measuring machine.
2. Discussion of Related Art
Coordinate measuring machines (CMM's) are in common use for dimensional inspection of workpieces. Typically, a workpiece is secured to a table, and a probe, such as one using a touch sensor, is moved in three dimensions on an arm of the CMM within a measurement volume to contact the workpiece at various points. When the probe contacts the workpiece, measuring scales in the x, y and z directions are read to obtain the position coordinates of the contacted point on the workpiece. By contacting various points on the workpiece, measurements of workpiece features can be obtained.
Probe assemblies used by CMM's often include an elongated probe shaft that is mounted to the arm of a CMM at a first end, and a spherical probe tip positioned at a second end of the probe shaft. The elongated shape of the probe shaft allows the probe tip to access recesses and difficult to reach areas of a workpiece. The probe shaft is often mounted to the CMM arm at one of several different angles, which allows the probe shaft to be oriented to access different portions of a workpiece and/or to approach a surface of a workpiece at a preferred or optimal approach angle.
The probe tip often has a spherical shape and a finite diameter, rather than being pointed or of another shape. A spherically shaped probe tip typically prevents the probe shaft from contacting a workpiece and allows a probe to approach a workpiece over a wide range of approach angles between the probe assembly and workpiece, when compared to a probe tip having a pointed tip. Spherically shaped probe tips may, however, make contact with a workpiece at different points on the probe tip itself, causing offsets that need to be compensated for when determining the actual point of contact as measured by the scales of a machine. Knowledge of the approach direction between the workpiece and probe tip, the diameter of the probe tip, and/or the general orientation of the workpiece surface being measured may be used to identify where on the probe tip contact occurred with the workpiece, and any offsets associated therewith.
A calibration routine and a calibration piece typically are used by a CMM to determine or to confirm the diameter of a probe tip. An accurate determination of probe tip diameter may be important to the accuracy of the measurements made by a CMM, since any inaccuracies in probe tip diameter measurements may render inaccurate measurements made by the CMM.